1. Field of the Invention
This invention relates to sensors used to detect the position or motion of an object with respect to another object, and to sensors for detecting such position or motion by utilizing displacement currents. In particular the invention relates to detecting the position or motion of a rotating object with respect to a bearing surface on which the object moves.
2. Prior Art
Many types of sensing devices are known which are capable of generating electrical signals in response to the relative motion between two or more elements. Examples of such devices are electromagnetic transducers, in which relative motion between an electrically conductive coil and a magnet produces electrical current signals; electret transducers in which relative motion between an electret member and another member produces an electrical current; piezoelectric transducers, triboelectric transducers and strain gauges. All electromagnetic transducers function in accordance with the principles enunciated by James Clerk Maxwell, usually expressed in the form of the well-known equation by Maxwell extending Ampere's Law: ##EQU1## where .mu..sub.0 is the permeability of air, .epsilon..sub.0 is the permittivity of air, .phi..sub.E is the electric flux, .beta. is the magnetic induction, and l is the length of a closed loop ohmic conductor. The quantity i is the conduction current flowing in the conductor 1, while the quantity ##EQU2## is termed the displacement current. This equation illustrates the interrelationship between a magnetic field and two electrical quantities: viz., the conduction current and the displacement current. The equation also shows that a changing electric field acts as a source for a magnetic field in the same manner as the conduction current acts as a source for a magnetic field. The conduction current is indicative of charges moving along an ohmic conductor, while the displacement current has the dimensions of a real current even though charges are not transported along an ohmic conductor. Thus, a magnetic field may be established in two ways--by a changing electric field (the displacement current term); and by a conduction current (the conduction current term).
In most known electromagnetic sensors and transducers a changing magnetic field generates the electrical current signal, and only the conduction current is sensed because the magnitude of the displacement current is negligible compared to the conduction current. While transducers which employ the conduction current are generally quite useful, certain limitations inhere in any transducer of this type. Because such transducers require an ohmic circuit for the conductive current, varying electromagnetic radiation in the vicinity of the circuit (which is usually coupled to amplifying and measuring circuitry) produces spurious conductive current signals which may mask the information conveyed in the conductive current signals generated by the transducer. Accordingly, shields are required to protect the circuit from stray electromagnetic radiation. Such shields make the structure of the transducers and the circuits undesirably complex. Conductive current sensors frequently require the application of electrical power to the circuit to enable the device to operate. The need for electrical power is particularly undesirable in applications requiring the installation of many sensors. In such situations either individual sources of electrical power must be installed at the site of each sensor or wiring must be provided from a central source to each sensor.